The "Customise this page" Icon can be seen in the top-right of the Dashboard Page and once activated enables the use of adding blocks to your own personal instance on the Moodle sites Dashboard area, any blocks added by this method will not affect any other user and is a great way to test blocks without worrying about changing users dashboards.
Customise this Page
Add a block
(For information regarding adding and removing Blocks please refer to the "Blocks" solutions Article) - Blocks Solutions Article Link
You can alter the Dashboard layout for all users by heading to:
Site Administration > Appearance > Default Dashboard Page
This will take you to the setup page where you can make changes by adding blocks or altering where different elements sit on the page, once you are happy with the layout you can press the "Reset dashboard for all users" button to make the changes instantly updated for all users accessing the dashboard page.